1. Field of the Invention
The present invention relates to graded reflectivity mirrors, such as are used for output couplers in unstable laser resonators; and, more particularly, to graded reflectivity mirrors with high effective reflectivity manufactured from multiple dielectric layers.
2. Description of Related Art
Unstable laser resonators are commonly employed to achieve transverse mode patterns that fill, or nearly fill, the available volume within a gain medium of a laser resonator for high efficiency, while suppressing higher order transverse modes. The typical output coupler for unstable resonators of the prior art is the "dot" mirror. These dot mirror output couplers have a low effective reflectivity because they use the spreading of the beam outside of the central reflective dot region on the output coupler as the coupling mechanism. This property has limited the usefulness of unstable resonator designs to laser systems with relatively high gain. See Siegman, LASERS (University Science Books, 1986) pp. 47-48; and U.S. Pat. No. 4,310,808, entitled HIGH POWER LASER EMPLOYING AN UNSTABLE RESONATOR, invented by Byer and Herbst.
An alternative output coupler of the prior art for unstable resonators is known as the variable, or graded, reflectivity mirror. Use of graded reflectivity mirrors (GRMs) as output couplers for unstable resonators greatly improves the output beam quality of the resonator. In these output couplers, rather than having a dot of high reflectivity with hard edges as an output coupler, a tapered layer of dielectric is formed on a substrate causing a radially variable reflectivity profile. The layer typically has an optical thickness of a quarter wavelength at the center and smoothly as possible tapers to a thickness of 0. The taper is generally assumed to cause a Gaussian reflectivity profile from a peak at the center to 0 reflectivity at the edges of the dielectric layer. See, for instance, De Silvestri et al., "Nd:YAG Laser with Multi Dielectric Variable Reflectivity Output Coupler", OPTICAL COMMUNICATIONS, Vol. 67, No. 3, pp. 229-232, Jul. 1, 1988 (and references cited therein).
GRMs also suffer the problem of relatively low effective reflectivity. The low effective reflectivity results mainly because materials suitable for manufacturing the mirrors do not have high enough reflectivity in the geometries commonly used, particularly in the visible and near infra red ranges. Therefore, GRMs have been limited, as have the dot mirrors, to systems with high gain. One such laser system is manufactured by Quantel International, Santa Clara, Calif. (Quantel's Datachrome line of Q-switched Nd:YAG Lasers). For lower gain systems, the GRM output coupler has not proved practical.
The effective reflectivity of GRMs can be improved by using a multi-layer mirror as described by Piegari et al., "Optical Coatings with Radially Variable Reflectance", FA16-1, pp. 406-408, Proceeding Office OSA, Topical Conference on Optical Through Filters, Tucson, April 1988, and Zizzo et al., "Fabrication and Characterization of Tuned Gaussian Mirrors for the Visible and Near Infra Red", OPTICS LETTERS, Vol. 13, No. 5, May 1988, 342-344.
However, the smooth reflectivity taper is lost for such multi-layer mirrors due to the interference fringes which cause rings of zero reflectivity in the mirror. FIG. 1 is a graph of the reflectivity of a mirror formed of three tapered dielectric layers. The graph shows the optical thickness of each of the three layers at trace 10, where optical thickness is equal to the index of refraction n times the actual thickness T. The reflectivity profile of the mirror as a whole is shown at trace 11. The mirror is formed on a BK-7 substrate having an anti-reflective coating on a first surface. A first dielectric layer of HfO.sub.2 is formed with the optical thickness profile of trace 10. On top of the first layer, a second dielectric layer of SiO.sub.2 is formed having the same optical thickness profile. Finally, a third dielectric layer of HfO.sub.2 is formed with the same optical thickness profile. Thus, the optical thicknesses of the three layers have profiles that start at 0.25 wavelengths at the center and continuously decrease to a thickness of 0 at the perimeter of about 1.8 mm radius.
For a single layer graded reflectivity mirror, the zero reflectivity perimeter would match the zero thickness perimeter of the dielectric layer. However, for the multi-layer embodiment of FIG. 1, an interference fringe at radius of approximately 0.6 mm (point 12 in FIG. 1) is induced. This forms a ring of zero reflectivity that effectively limits the size of the transverse mode that can be effectively coupled using the multi-layer GRM. This characteristic limits the usefulness of multi-layer GRMs of the prior art to laser systems with relatively small effective apertures.
The problem caused by the rings of zero reflectivity in multi-layer GRMs becomes more pronounced as the number of dielectric layers is increased. Therefore, the higher the reflectivity required and the more layers used to achieve that reflectivity, the smaller the effective aperture of the output coupler becomes. The small effective aperture complicates the problem of utilizing the optimum amount of the volume of the gain media.
Accordingly, it is desirable to provide GRMs for use in unstable laser resonator and other laser related applications that can be used with large aperture laser systems and systems with relatively low gain.